wireproto: add media type to version 2 capabilities response
This is useful to advertise because servers reject unsupported
media types. A client may wish to speak multiple media types and
choose the one the server supports.
I doubt we'll ever use multiple media types or negotiation in core.
But during the course of developing this protocol, I may end up
making extensions that backport and forward port protocol support
as needed to support Mercurial deploys in the wild. e.g. I may
deploy support for an older protocol on a server so old clients
can continue using it.
It's worth pursuing changing the SSH protocol's upgrade mechanism
to support multiple media types as well...
Differential Revision: https://phab.mercurial-scm.org/D3299
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